Re: [Discuss-gnuradio] OFDM packets

[Tom Rondeau]

Crespi Floriana wrote:
> Hi,
> I have a problem with OFDM system.
> I transmit OFDM signal at 2.4 GHz, but I receive half of the total packets.
> Moreover, only half of the packets received are correct.
> CRC controller is perfect but i want a ber estimate, so I change the
> payload (binary file) and I  receive check bit by bit.
> The result of my alteration test is similar than CRC test.
> I have a lot of frequency offset, therefore I change manually the frequency
> receiver.
> I use revision 6845 and RFX 2400.
> Example with crc:
> The final packet I receive:
> ok: False      pktno 4602   n_rcvd 1300       n_right 83
>
> Why can't i get all the packets?
> Could you help me??
>
> I'm testing OFDM system without USRP and none of the packets are lost.
>
> Thanks a lot.
>   

Crespi,

I'm not sure what state the OFDM code was in at that revision. That is 
likely a result of the timing issue I discussed in a previous email to 
the list.

The latest revision of the trunk has a much better implementation that 
works well over the air and perfectly in the loopback simulation. There 
is 1 minor problem left that I hope to get to over the weekend for the 
over the air operation. Remember, though, that we don't do any channel 
coding, so any wrong bit will cause a packet error. If you run the code 
currently over the air, BPSK is almost perfect while QPSK and QAMs will 
miss many packets due to only a few bit errors.

In general, the OFDM code is development work so it's likely to be 
buggy. We are working on it, but I can only spend a bit of my time with 
it currently. I would be very happy to see other people getting into the 
code and figuring out ways to make it better.

Thanks,
Tom